Lighting for industrial machine vision

Lighting plays a major role for industrial machine vision: Not the object itself, but only the visual image of the object is checked! It follows that only homogeneous lighting conditions result in a consistent image of the same inspection object. Fluctuating light conditions are therefore to be avoided at all costs!

Ideally, the lighting is used so that a perfect light-dark contrast is created:

Direct frontlight illumination

Surface partly very bright but overexposed, partly dark due to total reflection.

Diffuse dom light illumination

Surface smoothly illuminated due to homogeneous incident light.

Dark field illumination

Lateral edges are very bright. Scratches and dust are visible.

Backlight illumination

Part appears only as black silhouette. No surface information, contures ideal for measurement applications.

Only if it is possible to visualize the desired test characteristics or errors with enough contrast, these can then be evaluated with the image processing software. Usually the object is illuminated by an external light source. This principle sounds very trivial, but the practice reveals that it is one of the main difficulties of industrial machine vision to make the error in the object visible at all for the camera.

Some examples to illustrate the difficulties

• Imagine a transparent glass bottle, the embossed font in the bottom of which is supposed to be read. Test object and features are of the same material, and, what is more, they are transparent! The same applies to many embossed fonts on flat material surfaces.
• A scratch on a metallic surface, too, does often not cause any further alterations but a notch which has to be detected. Again a test situation where, in spite of the identical material, an error must be detected. The same applies to embossments and mouldings in materials.

Diffuse incident light

Diffuse light causes matte surfaces with low contrast of stamped font

Dark field

Low angle illumination causes emphasis on the edges and instamped font

The key role plays the light with its interaction in a functional chain of illumination, test object, filter, lens and camera:

Interaction between electromagnetic radiation and objects

Major factors influencing the image

As the object characteristics of inspected part cannot or can only hardly be influenced, the object determines the illumination and the camera! The camera, in turn, defines the optics due to its working distance and mount, due to its data format and data rate it also determines the image acquisition card.